They Are Billions

This isn't as hard as it sounds:

- Play on the frozen highlands level, as it provides the best natural choke points. You want to scout the area out, and if you do not like the level abandon, and try your luck again. You are looking for a level that closes off at least 1 side of your proposed colony entirely. Ideally 2. When I did this I got lucky with a god roll map that almost entirely closed off 3 sides of the map for me, forcing the three quarters of all entry points to come to me via a single choke point to the North, with two other entrances to the South and South East!
- Survive by focusing on army production as soon as possible, Rangers consume no food. In this level food production is at a premium, so go to town with Rangers for the early days of your campaign.
- The exception to this is to get inferno units as soon as possible, two of these guys supported by a dozen archers will melt all waves until the last few (make sure to keep your archers behind them at a safe distance 4 or so tiles behind the inferno units. Inferno units will BBQ your own army just as fast as they do the infected)
- Get the "all seeing eye" wonder (Eye of the Beholder) so that you can see the entire map ASAP.
- As you hit midgame, knowing the full layout of the map identify the 4 or 5 choke points that can protect your city once you reach endgame, and go to town with spike traps and barbed wire. I laid traps 20 tiles deep in some places. By the final wave you want everything to be metal wire traps rather than wooden spikes.
- As you approach endgame add a handful of Thanatos to each choke point for AoE damage and you will be amazed how fast everything dies.
- Again, as endgame approaches, try to put together a squad of titans (at least 5, ideally 8, more is a waste of resources imo), and keep them somewhere central ... these are your emergency responders, as each wave comes bring them to face the wave head on -- they will be able to stand their ground against the incoming horde in what is one of the most impressive sights in the game imo. For the final wave, use these guys to protect whatever you think is your least well fortified choke, but be ready to move them elsewhere at a moment's notice.
To summarise how things should be looking for the final wave:

- army divided up into 4 or 5 squads at your selected choke points
- each squad consists of at least 4 inferno, at least 4 thanatos, plus whatever assortment of rangers, soldiers, and snipers you have at this point (I had around 50 units per choke point, but that was overkill I think).
- each choke point is ~10-20 tiles deep in wire traps, you will be surprised how much of the horde falls before it even reaches your army, and what remains will be easy pickings

If it looks like you will be overrun, start knocking down buildings behind your defensive line before the horde gets too close that your not allowed to. You don't need energy or food to place more traps, and you should have a mountain of resources stockpiled anyway. Favour retreat and repositioning over losing troops, but don't panic when the horde reaches your inferno units, that's totally expected .. you should see that the inferno units are taking no damage even if they are surrounded by the horde, because the infected are being killed within milliseconds of entering their melee range thanks to the fire, the aoe, and the alpha strike from your other army units that are opening up as soon as the infected get within striking distance of the inferno units.